The collection of all living species inhabiting a particular area is called a community. The composition of a community changes over time, and this change is called succession.
Basic Knowledge
Community structure: dominant species(the species with the greatest biomass)・keystone species(has a major ecological influence even with a small population)・stratified structure(forest: tall trees・subcanopy trees・shrubs・herbaceous plants・mosses).
Ecological succession: primary succession=begins on bare land・lava plateaus, and other places without soil. Bare land→lichens・mosses→grassland→shrubland→sun-loving tree forest→mixed forest→shade-tolerant tree forest(climax). Secondary succession=begins after wildfires・logging, and other disturbances where soil・seeds remain. It is faster than primary succession. Climax=the final stable state of succession.
Sun-loving trees vs. shade-tolerant trees: Sun-loving trees(pioneer species)=prefer strong light・grow rapidly・dominate in the early stages of succession(examples: Japanese red pine・Japanese white birch). Shade-tolerant trees=grow even in weak light・grow slowly・dominate at the climax(examples: chinquapin・oak・Japanese beech).
Light compensation point=the light intensity at which the amount of photosynthesis=the amount of respiration(shade-tolerant trees have a low value). Light saturation point=the light intensity at which photosynthesis becomes saturated(sun-loving trees have a high value).
Why does succession stop when it reaches a shade-tolerant tree forest(climax) during primary succession? Explain based on the difference in light requirements between sun-loving and shade-tolerant trees.
Answer: The canopy of a shade-tolerant tree forest blocks light, so the forest floor is dim. Young sun-loving trees have a high light compensation point and cannot grow in weak light, whereas young shade-tolerant trees have a low light compensation point and can grow even in weak light. Therefore, only young shade-tolerant trees can grow on the forest floor, and the shade-tolerant tree forest is maintained through successive generations, becoming the climax.
- Primary succession=from bare land・without soil・slow
- Secondary succession=soil remains・fast
- Climax=final stable state of succession
- Sun-loving trees=pioneer species・require strong light(high light saturation point)
- Shade-tolerant trees=dominate at the climax・grow even in weak light(low light compensation point)
- Keystone species=major ecological influence even in small numbers
- Light compensation point=light intensity at which the amount of photosynthesis=the amount of respiration
Points to Note
① The difference between primary and secondary succession is the presence or absence of soil. ② When sun-loving trees grow and form a forest, the forest floor becomes dark, so young sun-loving trees cannot grow while young shade-tolerant trees can grow(sun-loving trees exclude themselves). ③ Climax communities may be disrupted by wildfires・typhoons and undergo secondary succession again.
Practice
- What is the final stable state of ecological succession called?
- Explain the difference between primary and secondary succession based on the characteristics of the places where they begin.
- Explain why shade-tolerant trees can dominate at the climax because of their low light compensation point.